In electron beam microfabrication it is essential to be able to accurately locate fabricated detail on the surface of a substrate. Several techniques have been proposed which involve the precise measurement of the substrate position. This invention is concerned with the measurement of position using a laser interferometer. In such an arrangement a reflecting device for example a mirror or retroreflecting prism is mounted on the moveable member and the distance moved by the device determined with respect to a remote interferometer head. By arranging two such reflecting devices at right angles it is possible to monitor the position of a moveable member in two directions corresponding to the X and Y axes, using two appropriately positioned interferometer heads also at right angles to each other.
The accuracy of measurements which can be obtained using a laser interferometer technique is very great provided that there is no unwanted movement constituent parts of the optical system. Deflections and displacements of a few microns can be significant and the necessary rigidity is difficult to achieve when the moveable member has to be mounted in a highly evacuated enclosure such as a vacuum chamber of an electron microscope column.